Episode 3 – Clear Zone | Talking Traffic

Topics: Clear Zone Concept

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Talking Traffic Episode 3 – Clear Zone

Hello and welcome to episode 3 of Talking Traffic for September 3, 2007. My name is Bill Ruhsam, and I run this podcast and its website, talkingtraffic.org. Today’s episode is about the phrase “Clear Zone” which most people haven’t heard of, but all drivers in the U.S. have experience with.

I have been asked, “why do we cut down all of the trees when building a roadway?” anyone who has driven US Route 3 from Nashua, NH to Bedford Massachusetts both before and after the recent widening will appreciate the difference in feel between a highway that is tree lined on both sides and one that is wide open across the shoulders and center median. There is a compelling reason for this rampant tree-cutting to occur. It is called the “Clear Zone” concept.

To boil it down, Clear Zone means that the space along the roadway shoulder should be forgiving of driver error. If you should drive off the edge of pavement, road builders should provide you with enough room to recover, or to allow you to stop before encountering an object that will cause serious harm. From a practical standpoint, in the forested parts of this country, that means cutting down trees that are inside the specified width of clear zone. Trees qualify as obstructions that may cause serious injury if struck.

Clear Zone doesn’t just apply to trees; it applies to everything which might be a roadside hazard. Guidesigns, street lights, speed limit signs, culverts, bridge abutments, and even guardrail are all considered obstructions. Any obstruction within the clearzone must be removed, or if that is not possible, protected. Signs, for example, can not be effectively placed outside the clearzone. If they were, you wouldn’t be able to see them. Instead they are mounted on supports designed to break in a predictable fashion, flying over your vehicle, rather than through the windshield. These supports are called, descriptively, breakaway mounts, and are found on every sign post from short stop signs to large interstate guidesigns. I have included a number of images with descriptions in the show notes. Poles that hold up street lights are also designed to break away, if struck. However, there are structures that cannot be designed to break away. Signal poles, large light standards, overhead guidesign bridges, and bridge piers are a few examples. You wouldn’t want a signal pole to come crashing down into the street, causing further damage, if someone accidentally hits it. Instead, these structures must be protected in some fashion to prevent vehicles from striking them directly. Concrete barrier and guardrail are two options for protecting you, the driver, from these immovable objects. An important thing to remember, though, is that guardrail, while being softer than what it is protecting, is still an obstruction and is going to hurt a lot if you crash into it. In the show notes, you’ll see an example of what can happen if you hit a guardrail with your vehicle.

Another part of the clear zone concept is the ability for a driver to recover back to the pavement, or to bring the vehicle to a safe stop. If the slope of the shoulder is steep, a vehicle driving off the pavement will inevitably end up at the bottom of the slope. If it is too steep, the vehicle may flip, which is not an optimal way to drive your car. To this end, any shoulder slope steeper than 3 to 1 (that’s 3 feet horizontally to 1 foot vertically) must be protected by guardrail or barrier. Even then a 3 to 1 slope is not considered recoverable. It is assumed that a driver leaving the road on a 3 to 1 slope will end up at the bottom of the hill, but should be able to bring the vehicle to a safe stop. For a recoverable slope, a 4 to 1 is the steepest allowed, with 6 to 1 or shallower preferred (of course you don’t want to go *too* shallow, or the shoulder won’t drain water, but that is another episode).

You will notice, driving along the highways of the U.S. that the clear zone concept is not uniformly implemented. Numerous locations have trees right up to the edge of pavement, large utility poles within 10′ of the road, or other safety encroachments. This is relatively common on older roads with narrow rights of way. However, any new construction that utilizes federal matching funds must comply with the clear zone or the project will lose its federal monies. As this can be up to 80% of the funding source for the project, clear zone is taken very seriously by planners. Of course, the most important part of the clear zone concept is that it provides a safer roadway for all travelers, regardless of who pays for the construction. As I mentioned in the last podcast, it is the responsibility of a professional engineer to uphold the safety of the traveling public at all times.

So, what did we learn today: That “Clear Zone” is a term applied to keeping the sides of our roads safe by removing obstacles, reducing the slope of the shoulder, protecting an obstruction with guardrail or barrier, or installing a breakaway mount on a post that is required to be inside the clear zone. We also learned that the clear zone is a requirement on new construction with federal money. You might get away with building a new road with local funds and ignoring clear zone, but I wouldn’t want to be the engineer that is sued in court after there is a fatal wreck.

Show notes for this episode can be found at www.talkingtraffic.org, including images of various breakaway devices and a graphical demonstration of why guardrail isn’t always your friend. This podcast is licensed under a Creative Commons, Attribution, No-Derivatives, Non-Commercial license, which means you may share this with anyone you like, but don’t change it, or sell it. Comments and suggestions are always appreciated and can be sent to me at Bill@talkingtraffic.org. The next episode, due to a request, will be on the subject of roundabouts, traffic circles, rotaries, and the difference between them. Until next time!

Episode Notes

Here are some examples of breakaway supports for signs:


This is an example of a large guidesign seen in many locations throughout the United States. The posts for this sign are structural steel, and if placed directly into the ground, would be a significant obstacle. Instead, the foundation footing is placed separately, then fastened to the sign post with bolts through an open flange, as seen in the right-hand image. This bolted assembly will come loose under the impact, allowing the post to hinge upward as the vehicle passes beneath the sign.

Square tubing is a common sign post throughout the United States. It is installed in two pieces, with the bottom-most piece anchored in the ground, then fastened to the top piece using a ninety degree bolt. This connect comes loose during and impact and the sign goes over the vehicle.

A typical pedestrian signal pole and its breakaway base.

The breakaway installation for wing channel post. You will often see this type of post driven directly into the ground.

This is an example of correctly implemented clearzone. You can see there are no obstructions extending out from the edge of pavement. The tree on the left side is outside the recommended clearzone width by about 10 feet.

This is an example of using guardrail to protect drivers from a sideslope that is too steep. A vehicle leaving the ramp on the right might possibly, without benefit of the guardrail, flip over.

This is what designers try to avoid. Large steel poles are not forgiving objects to hit. Unfortunately, in this case the pole is outside the clearzone.

Guardrail is designed to protect the driver from something worse, in this case, the possibility of a rollover. It is usually designed to direct the errant vehicle back toward the traffic stream, or at least to not permit it to continue any farther. All guardrails have ends, though, and if you hit the end, it can do bad things to you. This is an example of an incident known as “guardrail impalement”.